Halogen complexes bond formation

A mixture of bromine in the gaseous state and nitrogen was in contact with sodium azide which detonated, and was explained by formation of Br-Ns bromine azide, which like all compounds that have halogen-nitrogen bonds, is hardly stable. Thus, bromine was mixed with ammonia at ambient temperature. The mixture was then cooled down to -95°C and red oil was formed, which detonated when the mixture was heated and reached -67°C. It formed the following complex ... 

We shall show both from experimental evidence about gas-phase complexes and, to a lesser extent, from the results of electronic structure calculations that a parallel definition of the intermolecular halogen bond is appropriate The halogen bond is an attractive interaction between a halogen atom X and an atom or a group of atoms in different molecule(s), when there is evidence of bond formation. ... 

The mechanism of this reaction shows that excitation of the substrate gave an n,n triplet state, but this excited state was unable to dissociate the carbon-iodine bond. This was demonstrated by showing that the n,n triplet state, when sensitized by chrysene, did not produce coupling products. Probably, the reaction occurred in an excited a,a triplet state mainly localized on the carbon-iodine bond, and the interaction between this triplet state and aromatic compounds led to homolytic cleavage of the C-I bond with the formation of both a 5-thienyl radical and a complex between the aromatic compound and the halogen atom. The formation of this complex was demonstrated by the presence of a short-lived transient with Amax = 510 nm, showing a second-order decay kinetics and a half-life of ca. 0.4 (is in laser flash photolysis. The thienyl radical thus formed... 

Recently, the arylation of several specific primary amines have been studied because of the potential biological relevance of the products or products further downstream in a synthetic sequence. For example, cyclopropylamine was shown to be a viable substrate for the coupling under standard conditions [203]. Reactions of 7-azabicyclo[2.2.1]heptane have also been conducted [204] under relatively standard conditions, but with bis(imidazol-2-ylidene) as ligand. Complexes of this ligand and DPPF showed similar catalytic activities, which proved to be superior to those of most bis(phosphine)s. ortfio-Halo anilines were also studied, in this case to provide access to carbolines after use of the halogen as a means of effecting cycliza-tions by an electrophilic or reductive C-C bond formation with the other N-aryl group [205]. 

Many electrophilic agents have been employed for ring closure and the nature of the electrophile plays an important role. The mechanism of the electrophilic attack, however, has not been completely clarified13 15. Thus, in halogen attack, both the formation of an onium ion and a concerted mechanism with attack of the nucleophile on a halogen-double bond complex have been proposed for substrates containing an internal nucleophile. 

It is most rapid with the fluoride and very slow with the iodide. Experiments suggest that the reactions proceed through the formation of intermediate complexes rather than radicals the readiness with which they are formed must decrease with the electronegativity of the halogen. A sihcon-halogen n bond possibly leads to the complexing. 

Alkylcobalts form in stoichiometric reactions of pentacyanocobalt(III) hydride and alkenes. This reaction occurs both for halogenated alkenes such as tetrafluoroethylene and for alkenes that contain other electron-withdrawing groups such as carbonyls, nitriles and arenes as substituents (see Table 6) . The addition is regiospedfic, forming the more substituted alkylcobalt. Prior coordination of alkene to cobalt to form an alkene(hydrido)cobalt complex, an intermediate in hydrometalation reactions, is not important. This reaction is a radical process however, by NMR, additions of [HCo(CN)5 ] " to diastereomeric alkenes such as fumaric and maleic add salts lead to a cr-alkylcobalt by stereospecific cis addition of Co and H to the double bond . The overall reduction is not stereospecific. (r-Alkylcobalt bond formation proceeds by either a concerted addition or a rapid collapse of a radical cage. 

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